1. Field of the Invention
The present invention generally relates to a CMOS image sensor, and more particularly to a CMOS image sensor capable of improving anti-blooming and/or charge transfer.
2. Description of Related Art
A complementary metal-oxide-semiconductor (CMOS) image sensor has been widely applied to mobile applications. The CMOS image sensor may be applied to other applications such as automotive and security. Requirements for the automotive and security applications are quite different from that for the mobile applications. For example, blooming is highly undesirable in automotive and surveillance application. Blooming happens when a pixel is filled up with photo carriers and can no longer collect more electron/hole pairs during pixel exposure. A bright pixel will spreads to several other pixels in the neighboring region.
The road scene, especially at night, has a high dynamic range. The CMOS image sensor is thus required to have a good blooming control at ultra-bright region in order to ensure that the neighboring dimly lit regions are not washed out by the blooming charges. Otherwise, many details get lost and it is difficult to extract the information from the scene. Moreover, at high temperature operation such as in a car, a hot pixel could be filled up by dark current even in the dark. The adjacent good pixels will become hot by receiving the blooming charges.
Charge transfer is another requirement for the automotive and security applications. Pixel charge transfer speed is a key component of data bandwidth of the CMOS image sensor. If charge transfer is not complete, it will be manifested as undesirable dark/low light noise, which limits the minimum acceptable light level of the image sensor.
For the reason that conventional CMOS image sensors could not effectively solve blooming and charge transfer problems, a need has thus arisen to propose a novel CMOS image sensor with improved anti-blooming and charge transfer.